Process and apparatus for assembling and resin-encapsulating a heat sink-mounted semiconductor power device

Abstract

A die-stamped frame is fastened to a heat sink metal baseplate by wedging flexible tabs into receiving indentations of the baseplate while keeping the frame substantially in contact with the surface of the baseplate. The wire welding operations may then take place on the end of the fingers of the patterned metal frame while the same are solidly resting on the surface of the baseplate thus facilitating the welding. The backing-off of the metal frame from the surface of the heat sink baseplate takes place upon the closing of the mold used for encapsulating in resin the device. The injection of the resin and its solidification "freezes" the pins in the backed-off position imposed by the mold upon closing, thus ensuring the electrical isolation between the pins and the integral heat sink baseplate.

We claim: 
     
       1. A process for assembling and encapsulating in resin a die of semiconductor wafer having metallized pads for wiring connections, which comprises fastening together a patterned metal frame and a flat metallic heat sink baseplate, disposed in respectively parallel planes, by wedging fastening tabs of said metal frame into indentations formed on opposite sides of said heat sink, and successively cementing said die on said heat sink, connecting said pads to the ends of coplanar fingers of said patterned frame by means of metal wires welded thereto, placing the mounted assembly in a mold capable of defining a cavity for containing said metal heat sink, said die, said connecting wires and an inner portion of said coplanar fingers, which extend beyond the perimeter of said cavity by passing between perimetral mating surfaces of said mold and of a counter-mold, and injecting resin into said cavity for encapsulating the assembly while leaving exposed a surface of said heat sink baseplate and ensuring the electric isolation of each of said coplanar fingers from said metallic heat sink, characterized by comprising the following steps: fastening said metal frame to said heat sink and maintaining said coplanar fingers of the metal frame adjacent the surface of the heat sink;   welding said metal wires on the ends of said coplanar fingers contacting the surface of the heat sink;   placing the mounted assembly on a substantially flat mold provided with raised portions for supporting the heat sink by said surface thereof, the supports being arranged along the sides of the perimeter of the heat sink on which said tabs of the metal frame are anchored thereto;   closing the mold by means of a counter-mold having a substantially flat mating surface provided with a cavity capable of completely containing said heat sink, said counter-mold pushing said fingers while bending said fastening tabs against the surface of said flat mold thus spacing said coplanar fingers from the surface of the heat sink;   injecting an encapsulation resin in the cavity defined between the surface of said mold and the opposed surface of the heat sink and laterally by said counter-mold and, partially, by the inner face of said raised supports of the mold; and   solidifying the injected resin within the space comprised between said surface of the heat sink and said fingers of the metal frame before opening the mold and recovering the encapsulated device.   
     
     
       2. A process for assembling and encapsulating in resin a semiconductor power device incorporating a metallic heat sink baseplate, which comprises: fastening a patterned metal frame to a heat sink baseplate while maintaining the metal frame substantially in abutment with the surface of the baseplate;   completing wire welding operations on the frame resting on the baseplate;   placing the completed assembly into a mold and causing, upon closing the mold, the separation of the metal frame from said heat sink baseplate while yielding bending said fastening tabs; and   injecting and solidifying an encapsulation resin inside the mold before opening the mold and recovering the encapsulated device.   
     
     
       3. A method of encapsulating a semiconductor die inside a plastic package comprising: attaching said die to a base member;   attaching said base member to a conductive lead frame, said conductive frame including a plurality of conductive fingers extending adjacent said die and in contact with said base member;   attaching electrically conductive metal wires to said fingers, said fingers being in contact with said base member during said attaching step;   attaching said conductive metal wires to said die to provide an electrically conductive path from said die to said lead frame;   placing said base having said die attached thereto adjunct a first mold member, said first mold member having supports in contact with said base;   forming a mold around said base and said conductive fingers, said mold including a cavity for receiving said base member having said die attached;   moving said conductive fingers out of contact with said base;   injecting a plastic molding material into said mold, said conductive fingers being spaced from said base while said molding material is being injected into said mold;   solidifying said molding material within said mold, said fingers being retained spaced from said base by said solidified molding material; and   removing said encapsulated die from said mold.   
     
     
       4. The method according to claim 3 wherein said step of moving said conductive fingers out of contact with said base is automatically accomplished simultaneously with forming said mold, said mold including a flat surface that contacts said conductive fingers and forces them away from said mold is closed.